1. Field of the Invention
This invention relates generally to the deburring and precise chamfering of slots used in conjunction with gear teeth, spline teeth, dovetail slots, keyways, and other non-symmetrical shapes, internal or external, and more specifically, to the deburring and precise chamfering of gear teeth using a grinding wheel or cutter to provide a precise deburring or chamfering operation in a single feed or pass.
2. Background Art
Slots for gear teeth or other types of articles requiring chamfering or deburring are usually cut or milled in automated machines, sometimes referred to herein as hobbing machines. These machines operate on a usually cylindrical or disc-shaped gear blank by sequentially hobbing, milling, or broaching the gear teeth using a conventional commercially available machine tool. Examples of hobbing machines are described in U.S. Pat. Nos. 2,451,447, No. 3,130,642, No. 6,116,828, No. 4,961,289, No. 6,575,812 and No. 6,839,953. Most of these prior art machines operate on externally oriented gear teeth having essentially standard shapes.
When machining a work piece with gear teeth, dovetail slots, etc., the resulting teeth or slots have a burr and sharp edges following the initial hobbing, milling, or broaching operation. Accordingly, a second finishing operation is required to remove the burr and chamfer the edges of the work piece. It is generally undesirable to leave sharp edges on the work piece, because sharp edges and/or a non-uniform chamfer at the edges of the work piece causes stress risers or points on the work piece during its functional operation. These stress points usually are developed at the sharp corners or non-uniform chamfered work piece edges. Moreover, sharp edges or non-uniform chamfered work pieces can cause stress cracks leading to premature failure of the gear wheel or other part associated with the slot or gear tooth. Especially in aerospace applications, such as helicopters and airplanes, this can result in substantial risk of loss of human life. Chamfered edges on the corners of a workpiece generally produce an oblique angle and preferably the gear teeth surfaces are at a 45° angle relative to the slot at the end faces.
Methods have been developed by gear manufacturers to address the problem of asymmetrically chamfered work pieces. For example, manual chamfering of the front or back edges of the work piece is performed by a person operating a hand grinder, which is one conventional method that is often used. Alternatively, the end edges of the work piece are chamfered using a stand-alone machine, for example a James deburring machine of Broomfield, Colo. in the U.S. or a Gratomat type deburring machine and equipment available from Rausch GmbH & Co. KG of Haan, Germany, or alternatively, a conventional contour milling machine. Both of these methods have been found to be unsatisfactory, however, due to the non-uniform chamfering produced by the Gratomat Deburring machine and the labor and wastage costs associated with contour milling. Moreover, hand chamfering operations using a hand grinder, even by a skilled operator, result in slight imperfections in the uniformity of the chamfer, which produce asymmetric chamfers on the work piece that can and do develop into stress cracks, eventually leading to premature failure of the gearbox or other equipment having the slotted element.
These alternative deburring methods are for the most part inefficient, mostly unproductive, and unnecessarily costly because of personnel and labor costs. The problems noted with hand chamfering typically result in up to 10% scrap and substantial rework, up to 30%, of hand chamfered products. Thus, what is considered necessary is equipment and an automated deburring/chamfering process that enable the work piece to have both a front and rear face chamfer provided by a single unit, that automatically produces uniform chamfers in slots of varying shapes, sizes so that the work piece can withstand the increased stresses of, and can be used in the high rotational speeds found in, aircraft and other aeronautical or aerospace applications.
Another method for chamfering the edges of gear teeth is disclosed in the aforementioned commonly owned U.S. Pat. No. 6,939,093, wherein a second hob for providing a sequential hob chamfering operation to the gear teeth is described. The hob chamfering process described is a vast improvement from the hand chamfering, and the other alternative methods described above, as it can provide a uniform chamfer around the complete periphery of a gear wheel in a quick and efficient process. Nevertheless, the hob chamfering process of the '093 patent has limitations in use, because it is not possible to utilize hob chamfering for certain applications, for example, non-standard gear teeth configurations, keyway or dovetail slots, or for internally oriented gear teeth for an internal gear wheel.
Most standard automated chamfering or deburring machines provide chamfers on gear teeth that extend outwardly from the gear diameter, that is, the chamfering is performed on gear cogs that extend outwardly from the central rotational axis of the gear wheel. A different set of considerations arise when the gear teeth extend from an outer diameter of the wheel inwardly toward the center of the gear wheel, or when the gear teeth are of a non-standard gear tooth shape, for example, when the gear is an external gear having spline teeth, or the slots between the teeth are in non-standard shapes, for example, dovetail shapes, etc. In such cases, it becomes more difficult or impossible to control the movement of an automated arm to obtain the correct orientation of the types of chamfer hobs or contour milling machines heretofore used by the prior art devices because the range of motion is limited by the dimensions of the slots and the size of the hob providing the chamfer.
Accordingly, the invention as described below and claimed herein provides for a grinding wheel or cutter which produces uniform chamfers on external and/or internal work pieces having external or internal gear teeth, or for slots having asymmetric or other non-standard shapes, for example, dovetail shapes. The grinding wheel or cutter is capable of providing such a uniform chamfer in a smooth, single pass with the grinding wheel feeding into the slot from only a single direction.